Referring to FIG. 1, in a general real time system, when a external device 100, such as I/O device or bus, send an external interrupt request to an processing unit 101, such as CPU, DSP, the processing unit 101 immediately processes the Inter Service Routine (hereafter ISR) corresponding to the interrupt after receiving it.
However, when the external device 100 sends too many interrupts to the processing unit 101 to be processed immediately by the processing unit 101. If the processing unit 101 processes every interrupt requests and processes the corresponding ISR, sometimes, it is easy to reduce the processing speed of a CPU or a DSP. Thus, the system efficiency is decreased and the difficulty of debugging is increased to cause unpredictable interrupt and may reduce the system efficiency.